The invention relates to the field of in situ mining, and more particularly to a method of in situ mining using plural alternating rows of injection and recovery wells.
In situ mining has been recognized as an effective technique for recovery of minerals contained in an ore or mineral bearing stratum. Prior techniques for in situ mining have utilized one or more vertical injection wells drilled to the depth of an aquiferrous mineral bearing stratum for injecting a chemical leachant into the stratum to dissolve the desired minerals. In the prior art, a plurality of vertical injection wells, generally greater in number than the number of recovery wells, are arranged about each recovery well to pump out the leachant containing the dissolved minerals. The wells are arranged in what are known as "five-spot," "seven-spot," and "hexagonal" patterns with one recovery well surrounded by a number of injection wells.
Each vertical injection and recovery well generally includes a vertical screen disposed at the depth of the mineral bearing stratum to allow ready flow of the leachant fluid into and out of the recovery and injection wells.
A chief disadvantage of such conventional in situ mining techniques is that a large number of wells must be drilled to adequately cover a given volume of an aquiferrous mineral bearing stratum because each recovery well requires a plurality of associated injection wells. The cost of drilling, pipes, and pumping devices is proportionally multiplied when larger ore fields are mined. The high cost of drilling and mining marginal ore fields makes such mining economically impractical.
Conventional injection/recovery well patterns characteristically produce a declining volume of dissolved minerals recovered over the lifetime of the mining operation because toward the end of the operation, most of the minerals in the area directly between the recovery well and surrounding injection wells have already been leached. Pumping speed, and hence recovery volume, using conventional well patterns is low since one recovery well receives leachant under pressure from several injection wells.
Vertically oriented injection/recovery screens, as used in conventional in situ mining techniques, must be precisely placed within the ore-bearing stratum in order to insure maximum extraction of the ore-bearing layer (i.e., greatest injection/recovery well efficiency). However, placement of vertical well holes, especially in shallow formations at great depths (approximately 1,000 feet or more), can be difficult and time-consuming. In addition, vertically oriented injection/recovery screens do not "sweep out" a large area of the ore-bearing stratum since the stratum is generally horizontally aligned, whereas the screens are vertically aligned.
It is known to use horizontally well-branches or "drain-holes" to drain off undesirable aquifer (water-containing) regions which interfere with oil well drilling and oil recovery. However, heretofore such horizontal well branches have not been used for in situ mining of a mineral bearing stratum using a leachant fluid and plural injection and recovery wells.
It is therefore a primary object of the invention to provide an improved method of in situ mining of a mineral bearing stratum.
It is an additional object to provide an improved method of in situ mining utilizing horizontal branches which method requires substantially less drilling and pipe to recover a given volume of ore-bearing material than conventional in situ mining techniques.
It is a further object to provide a method of in situ mining having a substantially constant rate of mineral recovery over the lifetime of the mining operation.